1. Technical Field
The present invention relates to an inkjet head that ejects ink and a method for manufacturing an inkjet head.
2. Related Art
As inkjet heads used for inkjet printers, piezoelectric type inkjet heads and bubble type (thermal type) inkjet heads are known. These inkjet heads are provided with a pressure chamber that stores ink, and structured to push out and eject ink, by an actuator in the case of the piezoelectric type inkjet head, and by a bubble that is generated by boiling a solvent in the case of the bubble type inkjet head.
Pressure chambers of the inkjet heads, for example, in the piezoelectric type and bubble type inkjet heads, are generally formed in a silicon substrate (silicon wafer) by a semiconductor process. Reasons for forming pressure chambers in a silicon substrate are because silicon substrates (silicon wafers) are relatively readily processed, and therefore pressure chambers can be accurately fabricated. Also, in the bubble type inkjet head in particular, heaters can be readily formed on a silicon substrate, and the silicon substrate has sufficient heat resistance to tolerate heating by the heaters.
It is important to lower the defect rate caused by foreign matters and defects and improve the yield in order to lower the manufacturing cost in a semiconductor process that uses such silicon substrates (silicon wafers) as described above. The yield is greatly influenced by the chip size. When a desired number of nozzles is to be secured in a chip for inkjet head (hereafter also referred to as a head chip), which is an element for forming the inkjet head, the head chip becomes relatively large, compared to an IC chip, and therefore it is essentially difficult to improve the yield.
For example, an inkjet head with the currently highest dot density is provided with 600 dpi (at a nozzle pitch of 42.3 μm), and the size of a single chip composing this inkjet head is substantially large, compared to an IC chip. Accordingly, the number of head chips that may be obtained from a single silicon substrate (silicon wafer) is fewer, compared to that of IC chips. Therefore, in order to secure a greater number of non-defective head chips, it is necessary to improve the yield to a level higher than that of IC chips.
As described above, for example, in a piezoelectric type inkjet head, pressure chambers alone are formed in a silicon wafer, and other components such as actuators having piezoelectric thin films composed of PZT or the like are laminated on the silicon wafer, thereby assembling the inkjet head. This manufacturing method has been in the mainstream of assembling inkjet heads. However, this manufacturing method has a problem in the accuracy in processing components other than pressure chambers, and therefore its ability in achieving higher integration is limited.
In this regard, a manufacturing method by MEMS (micro electro mechanical systems), in which actuators and wirings to be connected to the actuators are directly formed on a substrate, has been developed in recent years. According to this manufacturing method, actuators and wirings are formed on a substrate, and then the same substrate is processed to form pressure chambers. Further, the substrate is divided into individual pieces (diced) depending on the requirements, whereby head chips that are components of inkjet heads are manufactured (see, for example, Japanese laid-open patent application JP-A-2004-6722).
In such a manufacturing method, when forming a piezoelectric thin film (piezoelectric film) composed of PZT or the like by a vapor phase method or a liquid phase method, the annealing temperature for crystallization may reach, for example, about 600° C. Therefore, the substrate is required to have a heat resistance at least at 600° C. or higher. Accordingly, the use of silicon (a silicon wafer) as a substrate is very practical, because there is no problem in terms of heat resistance, and the aforementioned advantage in which pressure chambers can be readily and highly accurately formed can be maintained.
However, this manufacturing method requires additional steps of forming actuators and wirings to be connected to the actuators on a silicon substrate, compared to the method in related art in which pressure chambers alone are formed from a silicon substrate, such that the processing on the silicon substrate is prolonged, and defects that may be caused by foreign matters and deficiencies would likely occur. As a result, the number of non-defective head chips that can be obtained from a single silicon substrate (silicon wafer) is not very high, in other words, a sufficient yield cannot be achieved. Therefore, non-defective head chips cannot be secured sufficiently in absolute quantity, as described above, and a reduction in the manufacturing cost has substantially been prevented.